Although particulate matter (PM) exposures have been linked with poor respiratory health outcomes, most of these studies have focused on air sheds with urban and industrial sources of PM2.5. Wood smoke-derived PM also contributes to ambient PM in these urban areas, and is the major source of PM in many US rural or peri-urban areas, as well as in many communities within developing countries. This study will focus on indoor air quality and clinically relevant changes in health effects among asthmatics living in homes whose primary heating sources are non EPA-certified woodstoves. The Primary Aim of this study is to assess the efficacy of residential interventions to reduce indoor PM exposure from woodstoves and the corresponding improvements in quality of life and health outcomes for asthmatic children. The study area for this project will be three rural communities in Idaho and western Montana, including one Indian Reservation. This study will use a three arm (Tx1, Tx2, and Tx3) randomized placebo-controlled intervention trial. The interventions will be at the household level, and exposure and outcomes will be assessed for one asthmatic child in each household. Households in Tx1 will receive inactive high efficiency particulate air (HEPA) devices and will serve as the placebo group. Households in Tx2 will receive a new EPA-certified woodstove, while households in Tx3 will receive active HEPA devices. The Secondary Aims of this study are to assess the impact of these interventions on residential PM2.5 exposures and other health outcomes. Secondary exposure outcomes measured prior to and following the intervention will include PM2.5 mass, chemical wood smoke markers on PM2.5 filters (including levoglucosan and abietic acids), and biomarkers of wood smoke exposure in urine and exhaled breath condensate. Secondary asthma-related health outcomes measured prior to and following the intervention will include peak expiratory flow (PEF) and forced expiratory volume in first second (FEV1), biomarkers in exhaled breath condensate (i.e., pH and nitric oxide), and frequency of asthma symptoms, medication usage, and healthcare utilization. To our knowledge, this will be the first randomized trial in the US to utilize a wood smoke intervention to assess the impact of the consequent reductions in indoor PM on health outcomes in a susceptible population. The results from this project will be translatable to other regions in the US and the world where biomass burning is commonly used for heating and cooking. Relevance: This study will determine if reductions in PM2.5 from residential woodstoves, a common source of indoor air pollution, will result in health improvements among asthmatic children. Clinically significant health outcomes will be measured. Results from this study will provide important information for implementing public health strategies for asthma management in the presence of this common exposure.